Relatively low energy electron beams have been used successfully for such applications as surface sterilization and the surface treatment of containers and other articles, materials or workpieces, as described, for example, in U.S. Pat. No. 3,780,308, of Energy Sciences Inc., the assignee of the present application. Bulk electron-sterilization techniques are disclosed in U.S. Pat. No. 3,779,796, of said Energy Sciences Inc. In such applications as packaging material sterilization, direct-current beam generators of the type marketed under the trademark "Electrocurtain," by said Energy Sciences Inc., have been employed; such low energy electron beam generation being described, for example, in U.S. Pat. Nos. 3,702,412; 3,745,396; and 3,769,600.
There are advantages, in some applications, as mentioned in said Letters Patent, in the use of repetitive-pulse-production of such relatively low energy electron beams with the aid of cold-cathode electron sources, and with capacitor-discharge pulsing techniques of the type previously used in other types of pulse generators, including the Marx-type capacitor storage-spark-discharge generators long-applied to high-energy physics systems, among the more recent of which is the pulsing of lasers, as described in Physics Today, April, 1975. (Also, E. Ault et al., IEEE J. Quant. Elec., Vol. 10, p. 624, on [1974]).
Among the considerations in applying such techniques to the problems of the present invention, however, are the very serious consequences of even temporary erratic pulsing or the missing of pulses, which, when occuring in a production-line sterilization application, for example, can result in the potentially dangerous effect of failing to sterilize at all, or improperly or inadequately sterilizing the workpiece as a result of poor beam uniformity, directivity and the like. A new level of reliability over prior uses of these pulse techniques in other applications is thus required for the purposes of the present invention. Further, prior systems using such techniques were often directed to laboratory and experimental applications which did not require the longevity of operation and industrial reliability underlying the commercial requirements of production-line sterilization and the like.
It has been determined that one of the keys to the unreliability (for present purposes) of previous pulse techniques of this character has been the absence of a sufficiently wide triggering range of the spark-discharge gaps. Previously, fixed gap trigger generators have operated at relatively narrow triggering ranges of approximately 15 percent below the self-breakdown voltage; or, where dynamic range variation has been required, with manual adjustments of gap spacing or by multiple triggered gaps, clearly unsuitable for production-line operation. Near the upper end of the triggering range, occasional prefires will cause low output voltages; while near the lower end of the triggering range, occasional misses occur. In accordance with the present invention, on the other hand, triggering range capacity has been extended upwards of about 30 percent--operation found necessary for long service industrial life time.
Among the novel pulsing circuit features of the invention, are significantly improved and tailored conductive shield structures for increasing the stray capacitance to ground along the capacitor stack, and large-area spark gaps of the "rail" type with novel trigger location and operation. Underlying the invention, moreover, is the discovery of a technique for obtaining a novel substantially linear depth-dose profile characteristic, and an intermediate region of operation thereupon, that startlingly renders the effects of the electron beam impulses significantly less sensitive to possible voltage variations during the pulse generation, thus promoting substantially uniform irradiation of surfaces and workpieces (sometimes herein generically termed "products") passing the apparatus.
Accordingly, an object of the present invention is to provide a new and improved process of and apparatus for the generation of relatively low voltage, energetic electron beam pulses which are not subject to the above-described limitations and disadvantages, but that possess the increased reliability needed for many industrial applications, such as sterilization, and that, in large measure is attained by a significantly increased triggering range and by operation in a most-favored region of a relatively low-slope depth-dose profile characteristic of the generated beam.
Another object of the invention is to provide a new and improved pulse-generating capacitor bank construction that allows for a greatly increased range of voltage variation within which operation of the system is permissible, and with a concomitant increase in reliability.
Other and further obects will be described hereinafter and are more particularly delineated in the appended claims.
In summary, however, the invention contemplates a process of and apparatus for the irradiation of objects by energetic electron particles wherein the reliability of pulse generation has been so greatly enhanced as to make such techniques available to a broader range of commercial applications. In one of its important aspects, the invention embodies a method of insuring the reliability of the production of repetitive impulses of electron-beam energy for production-line sterilization and similar purposes, that comprises, repetitively generating electric-discharge pulses; applying the pulses repetitively to draw electron beam impulses from a cold cathode to and through an electron-pervious window means; disposing the window anode adjacent a portion of a region along which products-to-be-electron-beam-irradiated are passed; adjusting the impedance presented by the cold cathode-window anode to the impendence presented by the pulse generating step to produce a substantially linear electron-beam dose versus penetration depth characteristic curve of relatively low slope in the region near the one-half dose region of said characteristic curve, thereby to reduce the sensitivity of the electron beam impulses to possible voltage variations during the pulse generating step in order to insure substantially uniform irradiation of the products passing along said region. Preferred details are hereinafter presented.